Scrub the potatoes clean and cut into bite-size pieces. Boil the potatoes in water until fork tender. Drain the potatoes. Place the potatoes in a large bowl and keep warm. Sauté the onions in the oil until softened. Stir the flour into the oil and whisk around for about a minute. Add the milk. Cook and stir until thickened. Remove from the heat and whisk in the sour cream, lemon juice, salt, pepper, and sugar. Pour the sauce over the drained potatoes and sprinkle with fresh parsley before serving.